In most cases you should be able to trim a Francis Vane impeller, though there may be something about a particular design preventing it - for instance the material may prevent machining, like a 28% chrome cast iron. There's also usually less range of hydraulic coverage by impeller trim on a francis vane than a purely radial.
As for the affinity laws, they're still classified as radial flow impellers with specific speeds below 4000, so they could generally follow the laws. While they should be pretty accurate for speed changes, I don't think I'd trust them for impeller trim changes. Depending on the specific speed, the vane geometry can change pretty dramatically as you trim down the outer diameter, which becomes more prominent the closer you get to 4000 specific speed.
A trim will follow affinity laws but not if one trims too great a percentage.
If the distance between the impeller outlet and the volute gets to be too great the liquid will start to recirculate in the pump and this will cause additional losses.
In any case you better ask the manufacturer to calculate the trim for you. If they will not trial and error is the way to go.
I recollect seeing a general recommendation how much the impeller can be trimed depending on specific speed; i think that the pump engineering book was either by Alexey Stepanof or Igor Karrasic (not sure about spelling).
my guess is that the max. trim will be no more than 10%.
when trimming impellers, the similarity laws do not apply.
The reason is that they are for a geometrically similar range of impellers. When an impeller is trimmed the ratio of the overall diameter to the inlet diameter (for example) changes, so the affinity laws do not accurately reflect the change.
The good news is that any decent book on pumps tells you the expected change in operation for a trim. The effect of a trim is in general less that what the affinity laws would lead you to conclude.
I second your statement. Real life is far more complex than the affinity laws lead us to believe. That is why so many times I mention trial and error approach.
In an impeller trim case I would simply calculate the trim. Perform anywhere from 50% to 75%, assemble and test to see the influence of the trim that was perormed.
The reason why I mention to contact the manufacturer is because manufacturers know how effective a trim will be and they can get you the exact amount to be trimmed immediately.
Now to boost manufacturers a bit more. Simply ask the manufacturer to calculate the trim and buy a new impeller from them at the trimmed diameter. As a pump user you will save at least one or several disasembly / assembly and test sessions, not to speak of unwanted shut-downs that could ocurr.
